Let $\pi (x)$ be the number of prime numbers less than $x$. If $p-q=2k$ for fixed $k$, then:
$$\pi^{-1}(n)-\pi^{-1}(m)=2k$$
We apply Taylor development:
$$\frac{1}{\pi' \circ \pi^{-1} (c)} (n-m)=2k$$
But $\pi (x)\sim li(x)$, $li (x)=\int_2^x \frac{dt}{ln(t)}$ the integral logarithm of Gauss.
$$ln(\pi^{-1}(c))\sim \frac{2k}{(n-m)}$$
$$c \leq \pi(e^{2k})$$
Thus we deduce the twin prime number conjecture for $k=1$.